Just add axiom.

"These ... considerations, however plausible, cannot be considered a rigorous proof of .... This proof also cannot be derived from the ... axioms stated thus far. On the other hand, the ... is of very great importance for the entire mathematical .... Therefore, it has to be introduced as a special axiom, which, for reasons to be explained later, is called the ,,, axiom."

From a mathematics book with some text replaced by ...